In natural gas production facilities, it is often necessary or desirable to periodically or continuously inject liquids into a high pressure gas pipeline. One example is the injection of methanol to prevent any water present in the natural gas from freezing. Such liquids are injected by means of pumps which overcome the pressure of the compressed gas to force the liquid into the pipeline. These injection pumps are often powered by pneumatic devices, particularly in remote locations. In some situations, the compressed gas flowing in the pipeline is used to drive the pump, but usually only after it has been regulated down to a pressure suitable for the pneumatic device (often around 10 pounds per square inch). The exhaust gas from the pneumatic device comes out of the device at a lower pressure than the gas in the pipeline, so it cannot be reinjected into the pipeline unless it is first compressed. Therefore, the exhaust gas is usually vented to atmosphere. In some situations a gas such as propane is brought to the site, stored in a pressure vessel, and used to drive a pneumatic device. This gas is also vented to atmosphere from the pneumatic device.
This venting of the exhaust gas to the atmosphere is a problem, firstly because it is a waste of valuable gas, secondly because it causes environmental contamination. In the case of sour gas wells (i.e., wells producing natural gas with high hydrogen sulphide content), it is generally prohibited, on environmental and health grounds, to use drive apparatus actuated by well gas where the exhaust gas is vented to atmosphere. Accordingly, there is a need for drive apparatus for driving injection pumps and other equipment associated with natural gas wells, using raw gas from the well to actuate the apparatus, but without venting the actuating gas to the atmosphere.
U.S. Pat. No. 6,336,389, issued Jan. 8, 2002 to English et al., discloses one example of prior art apparatus directed to this objective, mobilizing the kinetic energy inherent in the differential pressure between areas of higher and lower pressure in a pressurized gas system such as a pipeline. The English apparatus uses a single-acting piston that reciprocates within an open-ended cylinder inside a pressure vessel, where the interior of the pressure vessel is in fluid communication with the area of lower pressure, such that the bottom end of the piston is always exposed to the lower pressure. A switching valve allows gas from the area of higher pressure to flow into the chamber at the closed end cylinder, thus inducing a pressure differential between the two ends of the piston, causing the piston to move in a downward or power stroke. Linkage mechanism is provided for transferring the energy from the power stroke to an oscillatingly rotating output shaft, which is then connected to an injection pump or other type of equipment to be driven.
At or near the end of the downward stroke, the switching valve opens the piston chamber to the interior of the pressure vessel and closes off flow or higher pressure gas into the chamber, thus equalizing the pressure on each end of the piston. Biasing means such as a spring then moves the piston back to the top of the piston, thus exhausting the gas in the piston chamber into the pressure vessel and, effectively, into the area of lower pressure within the pressurized gas system. At or near the end of this exhaust stroke, the switching valve closes off the piston chamber from the interior of the pressure vessel and opens the chamber once again to the flow of gas from the area of higher pressure, thus readying the apparatus for the next downward power stroke.
The English apparatus effectively provides means for gas-driven actuation of injection pumps or other equipment without venting of the actuating gas. The English apparatus can operate with pressure differentials as low as 25 psi, so the internal mechanisms of the apparatus are not exposed to high pressures, even though the pressure in the gas system that drives it may be 1,000 psi or higher. However, the output of this apparatus is limited to an oscillating rotary drive. Commonly-used chemical injection pumps, on the other hand, require a reciprocating drive. Accordingly, the use of the English apparatus to drive a reciprocating-drive pump entails some kind of motion-converting mechanism to convert the oscillating rotary output motion to a reciprocating motion. This adds to the overall cost and mechanical complexity of the apparatus used to drive the pump, and reduces the overall mechanical efficiency of the apparatus.
Since the English apparatus uses a single-acting piston, and thus produces power only on half of the piston strokes, its mechanical efficiency is less than would be the case for apparatus using a double-acting piston and producing power on each piston stroke. An additional drawback of the English apparatus is that the spring or other biasing means (for returning the piston to the top of the cylinder after each power stroke) must be compressed during each power stroke, thus consuming part of the energy inherent in the pressure differential and thereby reducing the power output of the apparatus.
U.S. Pat. No. 6,694,858, issued Feb. 24, 2004 to Grimes, discloses a gas-driven reciprocating drive unit that uses a double-acting piston within a closed cylinder, in association with a pressurized gas system such as a gas pipeline. A switching valve directs gas from area of higher and lower pressure to opposite sides of the piston. The pressure differential between the two ends of the double-acting piston causes the piston to move toward a first end of the cylinder, simultaneously exhausting the gas in the first end of the cylinder back into the pressurized gas system. A drive link connected to the piston is used to transfer the power generated by the movement of the piston to a pump or other piece of equipment. At or near the end of each piston stroke, the switching valve reverses the connections to the areas of higher and lower pressure in the pressurized gas system, thus inducing a pressure differential that causes the piston to move in the direction opposite to the previous stroke and thereby exhausting the gas in the second end of the cylinder back into the pressurized gas system.
One of the significant drawbacks and disadvantages of the Grimes apparatus is the susceptibility of the piston seals to wear and deterioration. In order to maintain a pressure differential between the ends of the cylinder, the double-acting piston requires circumferential seals of some suitable type to prevent the flow of gas between the two ends of the cylinder via the annular space between the piston and cylinder. The ambient pressure within the annular space between the seals is constant, and typically atmospheric (i.e., approximately 15 psi). In contrast, the gas pressure within each end of the cylinder may be 1,000 psi or greater. As a result (and unlike the piston seals in the English apparatus), both of the seals in the Grimes apparatus are continuously working against a very large pressure differential, notwithstanding the fact that the piston itself is exposed to only a small pressure differential. The high differential pressure acting across the seals induces proportionately higher friction forces at the cylinder interface. These friction forces must be overcome in order for the piston move, and the power required to do this directly reduces the available power output from the apparatus. If the friction forces become too high, the piston may be susceptible to seizing or stalling (“stiction”). In addition, the high friction forces promote wear on the seals, thus making seal replacement necessary more often than would be the case in absence of high differential pressures across the seals.
For the foregoing reasons, there remains a need for reciprocating drive apparatus that not only may be actuated by raw pressurized gas from a natural gas well without venting the actuating gas to the atmosphere, but that also provides a direct reciprocating final drive output without need for motion-converting mechanisms. There is a further need for reciprocating pneumatic drive apparatus in which the seals between the piston and cylinder of the apparatus are exposed to a low pressure differential, therefore being less susceptible friction-induced power output losses, and less susceptible to wear and deterioration, than in prior art pneumatic drive apparatus. The present invention is directed to these needs.